A “pilot bearing” bit might be “ball bearing” or “metal bushing”.
With regard to router bits the term to which you inquire generally refers to how the bit itself accommodates the rotation of the cutting edge with regard to a reference surface. It could be a curved or straight edge, like a round or square tabletop. The “bearing” rides along the edge and allows the tool edge to cut its’ profile along same, following any contour or straight it finds. The depth of the cut may be adjusted by various means.
A “bushing bit” is usually smaller in diameter than bearing guided bits and is used in applications where a bearing guided bit will simply not fit due to the inherent size of the pilot bearing.
I think that the confusion you might be experiencing is with the lexicon due to the dual nature of the term “bearing” with regard to your understanding of router bits. It is both expressed (and sized) as a noun but is visualized as verb. The (nominitive) bearing, the part that accommodates the rotation of the bit and allowing it follow a path, is (verb) bearing on the work.
As regards a bearing guided bit, not all are “ball bearing”. Not all are “bushing bearing”. But all “bear” on an edge. A ball bearing bit uses, well, . . ball bearings and a description of this is well within googling reach. Likewise bushing bearings. Bush bearings are very simple and generate a lot of heat and as such have limited application in fine woodworking (some would say the same about routers, but not me).
There is a ton of information and opinion on this site and others regarding this topic. Good luck in your project and welcome to Lumberjocks..